In recent years, studies on a number of biomarkers for cancer diagnosis and treatment have been conducted. Among them, cancer cell-specific membrane proteins are considered as the most appropriate biomarkers as they are often shed in the body fluids in detectable amounts and clinical presentation in body fluids is always preferred over invasive methods of biopsy sampling. Besides, membrane proteins are also attractive because of their potential use in cancer imaging and targeted therapeutic strategies.
Meanwhile, the field of biomarker identification primarily relies on 2D gel electrophoresis (2D-GE) mass spectroscopy. However elucidation of membrane proteins using this technique has a significant limitation. Only 30% of the total cellular proteins are from membrane and of this only 5% can be detected using 2D-GE. Therefore as an alternative, specific probes has been developed for membrane proteins that can be used as tools to identify the bound target.
In the last decade, there has been a renaissance in the field of nucleic acid probes-aptamers; that can bind to the target protein with high affinity and specificity. Particularly, aptamers which are isolated using the technique SELEX (Systemic Enrichment of Ligands using Exponential Enrichment) have been developed for a number of disease-associated proteins, and many of them are currently in clinical trials as therapeutic moieties teemselves or as tools for imaging or drug delivery (Lee, J. F. et al., Curr Opin Chem. Biol., 10(3): 282, 2006; Gilbert, J. C. et al., Circulation., 116(23): 2678, 2007).
Aptamers can be selected against complex targets, that is, live cells and tissues, using the cell-SELEX technique in addition to the prior SELEX technique (Guo et al. Int. J. Mol. Sci., 9(4): 668, 2008). The cell-SELEX technique has an advantage in that it allows the development of aptamers for diseased cells even when surface marker targets are unknown. In addition, the cell-SELEX technique is more advantageous than the prior SELEX process, because target proteins cannot show their original properties in their isolated state, and thus target proteins which are in a physiological state allow a more functional approach during a selection process. Thus, an ssDNA aptamer for tenascin-C was first developed using the cell-SELEX approach (Daniels et al., Proc. Natl. Acad. Sci. U.S.A., 100(26):15416, 2003), and then a PTK7 (protein tyrosine kinase) aptamer for acute myeloid cells (Shangguan, D. et al., J. Proteome. Res., 7(5): 2133, 2008) and a DNA aptamer for small cell lung cancer cells (Chen H. W. et al., Chem. Med. Chem., 3(6): 991, 2008) were also developed. Using substractive SELEX DNA aptamers have been synthesized that can bind to differentiated PC12 cells but not parental cells thus are useful in clinical diagnosis (Brand, R. & Mahr, C., Curr. Gastroenterol. Rep., 7(2):122, 2005; Lee, M. X. & Saif, M. W., Jop., 10(2): 104, 2009). However, the Cell-SELEX technique requires limited control by an optimized and specific selection process because of the complexity of cell surface proteasomes. Thus, a proper negative selection process is essential for the success of the Cell-SELEX process.
Meanwhile, pancreatic adenocarcinoma is the 14th common cancer worldwide and the 4th leading cause of cancer related deaths in US alone. Around 90% of these pancreatic tumors are ductal adenocarcinomas (PDAC)s (Bardeesy, N. & DePinho, R. A., Nat. Rev. Cancer, 2(12):897, 2002). It is a highly aggressive malignancy with a very low median survival rate. The high rate of mortality associated with is attributable to poor prognosis and profound resistance to conventional chemotherapeutic measures (Koliopanos, A. et al., Hepatobiliary Pancreat. Dis. Int., 7(4):345, 2008). Only 15-20% of such tumors are resectable and limitation of early diagnostic markers pose a major problem in its timely detection (Brand, R. & Mahr, C., Curr. Gastroenterol. Rep., 7(2):122, 2005; Lee, M. X. & Saif, M. W., Jop., 10(2):104, 2009). Thus, there has been a need for the development of a novel pancreatic biomarker which can promote early diagnosis and assist in the development of effective therapeutic agents.
Accordingly, the present inventors have made extensive efforts to isolate a pancreatic cancer-specific cancer which can be used for the early diagnosis and treatment of pancreatic cancer. As a result, the present inventors have selected an aptamer, which specifically binds only to a pancreatic cancer cell line, by the cell-SELEX (Systematic Evolution of Ligands by EXponential enrichment) process, and have found that the selected aptamer binds specifically only to a pancreatic cancer cell line without binding to normal pancreatic tissue, thereby completing the present invention.